High reverse breakdown voltage and low on-resistance are required for high current switching elements. Field effect transistors (FETs) that use a group III nitride-based semiconductor are excellent in terms of, for example, high breakdown voltage and high-temperature operation because of their wide band gap. Therefore, vertical transistors that use a GaN-based semiconductor have been particularly receiving attention as transistors for controlling high power. For example, PTL 1 and PTL 2 propose a vertical GaN-based FET whose mobility is increased and whose on-resistance is decreased by forming an opening in a GaN-based semiconductor and forming a regrown layer including a channel of two-dimensional electron gas (2DEG) on the wall surface of the opening. In this vertical GaN-based FET, a structure including a p-type GaN barrier layer disposed directly below an n+ source layer is proposed in order to improve the breakdown voltage characteristics.